1. Field of the Invention
The invention is directed to optical communication and, in particular, to an optical fiber mode scrambler.
2. Background Information
The maximum number of modes any optical fiber can propagate depends on the geometry/composition of the optical fiber and the wavelength of the optical source. The actual number of modes that do propagate depends on, among other things, the launch conditions from the optical source to the optical fiber.
There are two types of mode-distributions that have practical applications when working with multimode optical fiber. The first type of mode-distribution is “restricted launch”, where only a small sub-set of propagating modes is coupled. Restricted launch has the advantage of resulting in reduced differential mode delay and, hence, less optical fiber dispersion. The second type of mode distribution is called an “overfilled launch,” where optical power is coupled into as many possible propagating modes as is feasible.
There are advantages of an overfilled launch condition, whose product is referred to as a mode scrambler. For example, an overfilled launch condition may be used to characterizing multimode optical fiber components.
There are various techniques and devices for generating an overfilled launch condition in a multimode optical fiber. For example, one technique is to inject a single mode optical signal into several kilometers of multimode optical fiber. Micro-bending induced mode coupling along the optical fiber length eventually results in an optical signal that has stable equilibrium of optical power distributed among many modes (multimode optical signal). However, several kilometers of optical fiber are required for this mode transformation (from a single mode optical signal to a multimode optical signal). As a result, this approach is not practical, especially in a laboratory environment, which is where most testing occurs. Moreover, using several kilometers of optical fiber merely to test a multimode device is expensive and bulky.
Another technique for generating an overfilled launch condition in a multimode optical fiber when initially launching from a single mode optical fiber is to concatenate a short segment of graded index multimode optical fiber followed by a step index multimode optical fiber followed by another short segment of graded index optical fiber. The step index optical fiber effectively provides a launch condition that fills up the mode volume of the second graded index optical fiber, thus providing the desired overfilled launch condition.
Mechanical mode scramblers also have long been used to generate a multimode optical signal. A single mode optical signal is launched from a single mode optical fiber into a multimode optical fiber. The multimode optical fiber is placed in the mode scrambler, which has corrugated surfaces to provide micro-bends in the optical fiber and redistribute energy into all the modes in the multimode optical fiber, resulting in the desired overfilled launch condition. The mechanical mode scrambler physically bends the optical fiber such that the angle of reflection between the optical signal and the core/cladding interface will be altered as the single mode optical signal passes through the portion of the optical fiber being bent. In this way, the single mode launch optical signal will be coupled into many more modes to approximate an overfilled power distribution in the multimode optical fiber. One such mechanical mode scrambler is the FM-1 Mode Scrambler available from Newport Corporation in Irvine, Calif.
Despite the advantages, this type of mechanical mode scrambler imposes intolerable strain on the optical fiber when physically bending the optical fiber to alter the angle of reflection. Bending stretches one side of the optical fiber and compresses the other. Because most optical fibers are comprised of glass or plastic, any strain on the optical fibers increases the risk that they will break. Tight bends in optical fiber can cause cracks, which can affect the optical signal traveling through the optical fiber, and will eventually lead to breakage of the optical fiber. A broken or cracked optical fiber will not properly transmit an optical signal.
Additionally, to effectively approximate an overfilled power distribution in the optical fiber, the mode scrambler bends the optical fiber many times in alternating directions. This makes the mode scrambler difficult to use, and because the tests are not repeatable, the device cannot be properly characterized. The mode scrambler also must be physically large enough to accommodate multiple bends.